Dresden 2026 – scientific programme

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MA: Fachverband Magnetismus

MA 40: Focus Session: Chiral phonons and crystals coupled to magnetic order II

MA 40.5: Talk

Thursday, March 12, 2026, 10:45–11:00, HSZ/0002

The phonomagnet: Spontaneous order of phonon angular momentum — •Maike Fahrensohn and Matthias Geilhufe — Condensed Matter and Materials Theory Division, Department of Physics, Chalmers University of Technology, 41258 Göteborg, Sweden

Magnetism arises from the collective order of electron spin and orbital angular momentum. Here, we introduce a new type of magnetism, which appears due to spontaneous order of the phonon angular momentum. We consider a Hamiltonian containing an interaction term that couples the phonon angular momentum to the electronic spin density. Using an imaginary-time Dyson expansion, we evaluate the spin partition function and obtain an effective phonon Hamiltonian.

To first order in the coupling strength, this approach yields a Zeeman-like term, which couples the electronic magnetization to the phonon angular momentum and leads to a spin-induced bias of phonon chirality.

To second order, we obtain a bilinear interaction between ionic angular momentum modes on different sites, mediated by the Matsubara spin susceptibility. Decomposing the tensor into isotropic and anisotropic parts, we identify emergent phononic analogues of the Heisenberg (isotropic), Γ-model (symmetric, anisotropic), and Dzyaloshinskii-Moriya (antisymmetric, anisotropic) interactions.

Similarly to the conventional Heisenberg model for electronic spins, our model exhibits a phase transition in which ions spontaneously develop angular momentum at elevated temperatures.

Keywords: phonomagnetism; phase transition; Heisenberg model; collective order